How are ballots cast and counted: paper, scanners and hand counts?

Votes are counted either by hand or by machines, such as optical scanners that read marked paper ballots. Where a ballot has only one contest and a few candidates, a hand count is often simplest. Where many seats are on the same ballot, computers are often used to give quick results. Either way, the paper ballot is what lets anyone check the answer later.

An optical scan system works like a test-marking machine. A voter marks a paper ballot, and the scanner's sensors detect black and white pixels in the marking areas. Its processor creates a tally for each candidate, and it usually stores an image of the ballot for later review. In a precinct-count system the ballot is read at the polling station, while in a central-count system ballots are collected and tabulated later at a central facility. Because the paper ballots and the electronic memories are stored, audits can check the images and tallies, and investigations or court challenges can examine them.

Scanners are not perfect. Hundreds of errors in optical scan systems have been found, from ballots fed in upside down to paper jams, blocked or overheated sensors and programming errors. But hand counting has problems too. Manual counts are usually accurate within one percent, and hand counting has been found to be slower and more prone to error than other methods. In New Hampshire towns, the average error in hand-counted candidate tallies was 2.5% in 2002, including one town off by up to 20%.

Some ballots cannot be counted on the night. In the US, a provisional ballot is used when there are questions about a voter's eligibility that must be resolved first, so the final tally can come one to two weeks later. In Britain, Canada, France, Germany and Spain, tallying may be done on the night itself.